Vigorous agitation of metals during solidification is known to eliminate dendritic structure and produce a semi-solid "slurry structured" material with thixotropic characteristics. It is also known that the viscosities of such material may be high enough to be handled as a soft solid. See Rheocasting, Merton, C. Flemings, and Kenneth P. Young, McGraw, Hill, Yearbook of Science and Technology, 1977-78. However, processes for producing shaped parts from such slurry structured materials, particularly on a continuous basis, present a number of problems. Such processes require a first step of reheating a slurry structured billet charge to the appropriate fraction of solid and then forming it while in a semi-solid condition. A crucible has been considered essential as a means of containing the material and handling it from its heating through its forming cycle. The use of such crucibles is costly and cumbersome, and furthermore, creates process disadvantages such as material loss due to crucible adhesion, contamination from crucible degradation and unwanted chilling from random contact with crucible sidewalls. Further problems are involved in the heating, transport and delivery of billets which are in a semi-solid condition.
Recognizing that it would be desirable to provide an apparatus and process for producing shaped metal parts from semi-solid preforms, the invention disclosed in U.S. Pat. No. 4,569,218 was made. In accordance with that invention, it was found that it is possible to produce on a continuous basis shaped metal parts from slurry structured freestanding metal preforms by sequentially raising the heat content of the preforms as they pass through a plurality of induction heating zones. The heating sequence is such that it somewhat avoids melting and resulting flow and permits thermal equilibration during transfer from one zone to the next as the preforms are raised to a semi-solid temperature. The invention further provides that the preforms take on the characteristic of being substantially uniformly semi-solid throughout their bodies. The freestanding semi-solid preforms are taught to be transferred to a press or other shaping station by means of a mechanical device which grips the preforms with a very low force, which both prevents substantial physical deformation of the semi-solid preform and reduces heat loss. As a preferred embodiment, U.S. Pat. No. 4,569,218 further teaches that the transfer means may be heated to even further minimize heat loss of the preforms during transfer.
More specifically, the apparatus disclosed in the above-referenced patent comprises, in combination, means for supporting and positioning a plurality of slurry structured freestanding metal preforms which include means for passing the preforms through a plurality of induction heating zones containing induction heating means for sequentially raising the heat content of the preforms while the preforms remain freestanding to a level at which the preforms are semi-solid. Means are then provided for transferring the freestanding preforms from the supporting means to a shaping means while the preforms remain in a semi-solid state, said transfer occurring without substantial deformation of the preforms and without substantial local variation in fraction solids within the preforms. Means are then provided for shaping the preforms while in the semi-solid state into a shaped metal part and means for recovering the solidified shaped metal part.
Although the invention disclosed and claimed in U.S. Pat. No. 4,569,218 is adequate, particularly when dealing with semi-solid preforms of rather small mass, as preform sizes have increased to produce larger and larger shaped metal parts, it was noted that the semi-solid preforms tend to excrete liquid metal. This tendency is particularly noticeable as the preforms pass through multiple induction heating zones and approach the appropriate temperature for shaping into the final metal parts. The loss of liquid can be related to the degree of melting which is induced by the induction heating elements and appears to be relatively unrelated to the rate of heating. Typically, a 1 kg slug of aluminum alloy 357, being of a cylindrical configuration and having a diameter of approximately 2.5" and a length of approximately 5" would lose from 50 to 100 grams of liquid when heated an appropriate amount for shaping into a forged part. This results in upwards of a 10% metal loss. Besides losing a significant portion of each preform, the liquid which emanates from each preform accumulates on the processing equipment, which must be cleaned after each cycle and results in operational inefficiencies.
It is thus an object of the present invention to provide an apparatus related to the invention disclosed and claimed in U.S. Pat. No. 4,569,218 while avoiding the liquid metal losses referred to above.